1. Field of the Invention
This invention relates to mid-range and high frequency audio speakers (tweeters), having back-cavities formed of a rigid, open celled, foamed material.
2. Description of the Prior Art
A tweeter is often mounted together with a low frequency speaker (woofer) in the same speaker cabinet. Such a tweeter is, to some extent, externally driven by the sound pressure vibrations generated inside the cabinet by the woofer. These high amplitude vibrations cause harmonic distortion and cross modulation distortion. In order to overcome this problem, conventional tweeters are provided with a closed cavity on the rear side to diminish the back-pressure effects caused by the woofer. Such a cavity is typically formed by pressed iron sheets, molded plastic, or paper and wood, and its strength is generally insufficient to adequately withstand the high amplitude pressures induced by the woofer. Furthermore, the cavity itself is vibrated to some degree by the woofer backpressure, and the tweeter diaphragm is thus driven by the air within the cavity, which leads to distortion.
In addition, the acoustic pressure generated inside the cavity by the tweeter itself creates standing waves and reflected waves, and the reaction of these waves with the speaker diaphragm lowers the frequency response and transient characteristics, and induces harmonic distortions. Heretofore, a sound absorbing material, such as glass wool, has been put in the hollow cavity to suppress the standing and reflected waves. Glass wool has a relatively high acoustic absorptivity at frequencies above 2,000 Hz; its acoustic absorptivity decreases sharply, however, at lower frequencies. The standing and reflected waves generated in the cavity depend on, inter alia, the dimensions of the cavity. In a typical example, where the cavity is 150 mm in diameter and 150 mm in depth, and the nominal diameter of the loud-speaker installed therein is 120 mm, these waves are generated at frequencies of approximately 800-1000 Hz. Thus, the conventional sound absorbing materials are ineffective to adequately suppress standing waves or reflected waves at these frequencies.